Journal of the Meteorological Society of Japan. Ser. II
Online ISSN : 2186-9057
Print ISSN : 0026-1165
ISSN-L : 0026-1165
Volume 76 , Issue 6
Showing 1-15 articles out of 15 articles from the selected issue
  • Hisayuki Yoshikawa Inoue, Masao Ishii, Hidekadzu Matsueda, Masayoshi Y ...
    1998 Volume 76 Issue 6 Pages 829-839
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    In order to measure the CO2 mixing ratios in air equilibrated with seawater (xCO2S), we tested a multi-layered composite hollow-fiber membrane (MHF) module aboard the TS Hokuto-maru in the North Pacific in January-February 1996 and July-August 1996. The MHF module is much smaller (300cm3) than the currently used shower-head type equilibrator (110dm3 installation volume). The mean difference in xCO2S (ΔxCO2S) values between the two systems determined 14 minutes apart was 1.6ppm with a standard deviation of 2.2ppm (n=73) for the southbound cruise in January, and -1.9ppm with a standard deviation of 1.8ppm (n=103) for the northbound cruise in February, 1996.
    The difference in average ΔxCO2S between the two legs was mainly caused by the effect of water temperature difference between the shower-head type equilibrator and the MHF module. By taking into account the effect on xCO2S of water-temperature difference between the two equilibrators, the mean ΔxCO2S was as low as 0.1ppm (n=732) with a standard deviation of 3.7ppm for the cruise in July-August 1996. These results indicate that no systematic difference exists between the two equilibrators. The MHF has potential for future use as an equilibrator that is compact and easy to install.
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  • A. Chandrasekar, Akio Kitoh
    1998 Volume 76 Issue 6 Pages 841-853
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    Observations indicate two favorable locations for the Tropical Convergence Zone (TCZ) during the Indian summer monsoon, one over the continent and the other over the equatorial Indian Ocean. An active spell of one TCZ coincides with a weak spell of the other TCZ. Observations also show the presence of positive sea surface temperature (SST) anomalies south of the equator over the Indian Ocean during the weak Indian summer monsoon years. The impact of such SST anomalies on the Indian summer monsoon is investigated through general circulation model ensemble experiments. The results indicate significant response over the Indian region and this response is manifested as a decrease in the monsoon precipitation and the weakening of the mean monsoon circulation. A series of identical experiments with a negative SST anomaly prescribed over the same region with the same magnitude confirm these findings.
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  • Kozo Ninomiya, Chiaki Kobayashi
    1998 Volume 76 Issue 6 Pages 855-877
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    The large-scale features of the moisture transport and the water balance of the Asian summer monsoon in 1991 were studied by utilizing the 24-hour prediction by a T106 spectral global model. In part I of the present study, we will describe the seasonal change of the precipitation in relation to the change of the major circulation systems over the Asian monsoon region.
    The most remarkable change in the onset period of the Asian summer rainy season over the Indian Ocean and the Southeastern Asia is the rapid displacement of the major precipitation areas from the equatorial zone to the Indochina Peninsula and Indian subcontinent. The notable change in the onset phase of the rainy season over East Asia is the formation of a precipitation belt, which corresponds to the Meiyu and Baiu frontal zone.
    In order to realize the relation between the variations of the precipitation with the those of the low-level circulation, several major circulation systems (CSs) in the lower troposphere are defined. Among them, the variations of the clockwise circulation centered over the Indian Ocean (CS-3) and a cyclonic circulation over the northern part of the Indian subcontinent (CS-6) are closely related with variations of the Indian monsoon rainfalls, while the cross-equatorial flow from the Australia anticyclone (CS-4) and the circulation around the North Pacific subtropical anticyclone (CS-5) are the major systems related with the East Asia rainfalls.
    The variation of the mixing ratio of water vapor, equivalent potential temperature, vertical stability and the vertical motion are also described in relation with the variations of the precipitation and CSs. During the Asian summer monsoon season, the gradual decrease of the mixing ratio and the increase of the vertical stability, in association with the subsidence, are significant features over the western tropical Indian Ocean, while the increase of the specific humidity and the vertical instability are the notable features over the Indian sub continent, Indochina Peninsula and the southern part of China.
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  • Miho Yamamori, Kaoru Sato
    1998 Volume 76 Issue 6 Pages 879-888
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    We examined medium-scale waves near the midlatitude tropopause and their background state in the framework of the quasi-geostrophic system.
    We made a 19-day continuous observation of the MU radar and radiosondes in April 1995. Using these observational data, we found that the active periods of the medium-scale waves are connected with extremely high values of poleward gradient of the stretching vorticity that are observed in the ridge phase of background synoptic-scale waves. Operational upper-air network data by JMA (Japan Meteorological Agency) showed that the characteristic vertical structure of static stability in the synoptic-scale ridge causes the large poleward gradient of the stretching vorticity within the latitude band of a few degrees.
    We also analyzed objective analysis data to examine horizontal distribution of quasi-geostrophic potential vorticity. Two kinds of meridional structure of the medium-scale waves are found: one has no node and the other has one node in the meridional direction. Both are located where horizontal gradient of stretching vorticity is large.
    These observational results are consistent with the theoretical consideration by Sato et al. (1998) that the medium-scale waves can be interpreted as internal modes trapped into the localized gradient of quasi-geostrophic potential vorticity near the midlatitude tropopause.
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  • Junichi Tsutsui, Akira Kasahara, Hiromaru Hirakuchi
    1998 Volume 76 Issue 6 Pages 889-907
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    Numerical experiments were conducted with a regional atmospheric model to investigate the suitability of the model for simulation/prediction of tropical cyclones (TCs). The model uses primitive equations and is similar in construction to a regional atmospheric model, called RegCM, which has been used extensively for regional climate modeling. The suitability of the model for TC study was tested in the mode of shortrange forecasts to find out how well the model can reproduce TC behaviors with a special attention on their genesis and development. Objective analysis data for typhoons Ed (9018) and Flo (9019) in September 1990 were used as the input data. In addition, “observed” precipitation (PR) rates estimated from hourly infrared satellite measurements were used.
    In the prediction experiments, we focused on two aspects: One was to study the impact of data initialization. The other was to examine the sensitivity of forecast depending on the choice of two cumulus parameterizations, Kuo and relaxed Arakawa-Schubert (RAS) schemes. The initialization to the input data was done through the combination of diabatic nonlinear normal mode initialization for dynamical fields and cumulus initialization for moisture. The diabatic heating rate was determined from radiative and condensation heating rates using the input data, including observed PR rates. The initialization of moisture was done through the inversion of cumulus scheme for a specified PR rate.
    Positive impacts of the initialization on reproducing the behaviors of both typhoons are clearly demonstrated in the use of both cumulus schemes. In particular, the initialization results in successful simulations of initial PR field without spinup problems. Thus, the positive impact is greatest in the case of TC genesis. The reproducibility of PR by the model with RAS scheme is superior, and the RAS scheme appears more suitable than the Kuo scheme for the intensity prediction of developing typhoons. However, the use of the Kuo scheme produces better scores in mean displacement errors.
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  • Hae-Kyung Lee, Pao-Shin Chu, C. -H. Sui, K. -M. Lau
    1998 Volume 76 Issue 6 Pages 909-923
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    In this paper, we describe the annual cycle in the latent heat flux (LHF) and its associated bulk variables (sea surface temperature, wind speed, humidity difference) over the equatorial Pacific. The in-situ, dailyaveraged TAO buoy observations between 8°N and 8°S during the period 1992-1996 form the database. LHF was computed using a modified bulk parameterization scheme to account for active convection and low wind speed frequently observed in the western Pacific. Harmonic analysis was used to help quantify the phase and amplitude of the annual and semiannual cycles.
    The annual cycle of LHF was found to be conspicuous in two regions, namely, the northeastern and western/central Pacific. For the former region, the maximum LHF occurs in boreal summer and early fall, when surface wind speeds are strong and the temperature difference between sea surface and air near the bottom of the atmospheric boundary layer is large. For the western/central Pacific, maximum LHF occurs in boreal winter, when the winter monsoon is strong. In contrast to the aforementioned two regions, the annual cycle in LHF in the equatorial cold tongue is weak and low LHF prevails throughout the year. Also noted in this study is a westward propagation of the maximum LHF region from the northeastern Pacific around July to the western Pacific by the following March.
    We also ascertained the relative importance of dynamic and thermodynamic processes in regulating the month-to-month variations of the LHF along two meridional transects, one in the eastern and another in the western Pacific. In the eastern Pacific, except to the north of the cold tongue, variations in humidity difference (i. e., thermodynamic process) seem to be of primary importance to the annual variations in LHF. On the other hand, variations in wind speed (i. e., dynamic process) are more important to the LHF in the western/central Pacific.
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  • Sung-Dae Kang, Fujio Kimura, Shunji Takahashi
    1998 Volume 76 Issue 6 Pages 925-935
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    We investigated formation mechanism of vortex streets in the lee of mountains by using a three-dimensional numerical model. The model is based upon the hydrostatic Boussinesq equations in which the vertical turbulent mixing is parameterized using the level 2.5 model of Mellor and Yamada (1982), but the horizontal viscosity is assumed constant.
    Kármán vortex streets can form even without surface friction in a constant ambient flow with uniform stratification. The flow in the lee of the mountain is decelerated due to divergence of the total vertical momentum flux associated with mountain drag. The divergence of momentum flux can be explained by the wave saturation theory given by Lindzen (1981) with some modification. Simulations in this study show that the momentum flux in the lower levels is much larger than the saturated momentum flux, whereas it is almost equal to the saturation value at the upper levels as expected from the saturation theory. This means that large flux divergence is produced between surface layer and upper levels (about 2.5km). As a result, the mean flow is decelerated behind the mountain and the horizontal wind shear forms. When the decelerated flow has a strong enough horizontal shear, the Kármán vortex will form due to an absolute instability as mentioned by Schär and Durran (1997).
    In case of a three-dimensional, bell-shaped mountain, the wave breaking occurs if the Froude number (Fr=U/Nh) is less than about 0.8, while a Kármán vortex forms if Fr is less than about 0.22. The results of the momentum budget calculated by the hydrostatic model are almost the same as nonhydrostatic results as long as the horizontal scale of the mountain is larger than 10km. A well developed Kármán vortex similar to the hydrostatic one was simulated in the nonhydrostatic case. Therefore, we conclude that the hydrostatic assumption is adequate to investigate the origin of the Kármán vortex from the viewpoint of momentum budget.
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  • Reiji Kimura, Junsei Kondo
    1998 Volume 76 Issue 6 Pages 937-953
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    A new canopy model having a simple parameter is here presented for estimating the diurnal or seasonal variations of the heat balance, employing meteorological data of temperature, wind, humidity, precipitation, and solar radiation gathered by AMeDAS near the experimental field. The calculated results were compared with observations over the rice paddy field for the two-year period of 1993 and 1994. Simulations using the presently developed canopy model agreed well with the daily-mean observed values. The rms error in the daily mean values between the observed and calculated values of surface temperature, sensible heat flux, and latent heat flux were 1.7°C, ±13Wm-2, and±16Wm-2, respectively, during the overall observational period. The annual evapotranspiration was evaluated as about 600mmy-1. This value is found between the values for shallow water (564mmy-1) and forests (692mmy-1). The ratio of rainfall interception to evapotranspiration during the vegetated periods are about 18% and 9% for the year 1993 (cool summer) and 1994 (hot summer), respectively. The precipitation amount in 1993 is large, while evapotranspiration from the vegetation is small, resulting in a large rate of rainfall interception.
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  • Wenjun He, Tetsuo Kobayashi
    1998 Volume 76 Issue 6 Pages 955-963
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    The evaporation of water from bare soil changes in mechanism as well as in magnitude as the surface dries. This evaporation proceeds in three stages, the third of which has a mechanism completely different from the other two stages. Therefore, only the parameterization schemes for soil-surface evaporation that take account of this stage switching should be considered as rational.
    A rational parameterization of the soil-surface evaporation based on a three-layer model of the vertical distribution of soil moisture is proposed. Each stage of evaporation is characterized by one of the three soil layers that comes to the surface. The switching from one stage to another can be determined from the difference between surface soil temperature and screen-level air temperature. The evaporation rate in the first stage is equal to the potential evaporation rate Ep, and that in the second stage can be expressed by the product of Ep and “surface moisture availability” M, which depends on the wetness of the superficial layer of soil and the soil type. Since these are practically the same as the existing parameterizations, only a general idea of formulating the two stages is given in this paper. The third-stage process, however, is formulated by a newly invented scheme that will be called the “DSL bulk method.” An example of the application of this method to the Tottori-Dune sand is also shown. This parameterization makes it possible to evaluate the evaporation rate in the third stage by the surface soil temperature and the average water content for the top 5cm of soil, and both can be measured by remote sensing.
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  • Toshiki Iwasaki, Hiroto Kitagawa
    1998 Volume 76 Issue 6 Pages 965-982
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    The influence of aerosol and cloud radiations upon the Asian summer monsoon was studied by using the global numerical weather prediction (NWP) model at the Japan Meteorological Agency (JMA). We parameterize both the direct and indirect effects of aerosols on radiative processes. In addition, criteria of cloud diagnosis are modified to enhance the cloud cover over the land in contrast to that over the ocean, considering that clouds form more easily over the land due to surface inhomogeneity and stay longer in the atmosphere due to smaller droplet size over the land than over the ocean.
    We confirm that the prediction of Asian monsoon activity is very sensitive to inclusions of aerosols and land-cloud enhancement. The control model, which uses the same cloud diagnosis scheme both over the land and over the ocean and does not include any effects of aerosols, systematically overpredicts the absorbed solar radiation (ASR) over land. The test model with these processes fairly reduces the systematic errors of land-ocean contrast of ASR. Over the Eurasian continent, the test model reduces ASR and lowers low-level temperature through land-atmosphere interactions. It suppresses monsoonal circulations in Southeast Asia and significantly delays northward migration of the typical fronts around East Asia, such as the Meiyu, Changma and Baiu fronts.
    The impact of land-cloud enhancement on one-month forecasts of the Asian monsoon seems to be similar to that of aerosols. Although the reduction of systematic errors of ASR indicates the relevance of the total effects of the two parameterization schemes, relative magnitudes of their impacts in nature are still uncertain. Further studies are required on distributions of aerosols and their optical properties, and cloud formation mechanisms particularly associated with the land-ocean difference, including aerosol-cloud interactions.
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  • H. L. Tanaka
    1998 Volume 76 Issue 6 Pages 983-1008
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    In this study, we conducted a series of numerical experiments to investigate atmospheric blocking, using a simple barotropic model that featured a wavemaker to excite synoptic disturbances. The model has a resolution equivalent to R20 and consists of only five physical processes: a wavemaker as baroclinic instability, topographic forcing, biharmonic diffusion, zonal surface stress, and Ekman pumping.
    Results of time integrations show that persistent dipole blockings appear one after another in the model, showing a reasonable life-cycle. In the model atmosphere, the synoptic disturbances are amplified exponentially by the wavemaker. The exponential growth soon saturates with nonlinear scattering of energy from synoptic to planetary waves associated with a Rossby wave breaking. The analysis of potential vorticity (PV) indicates that the onset of blocking is brought on by the Rossby wave breaking. The overturning of high and low PVs tends to occur at the topographic stationary ridge.
    Once a block is formed by the Rossby wave breaking, subsequent Rossby waves are blocked and undergo meridional stretch. The stretched wave then breaks down, depositing fresh low PV at the north and high PV at the south of the blocking system to maintain the block. The result is consistent with the so-called eddy straining mechanism. The result suggests that the exponential growth of synoptic disturbances is essential both for the onset and the maintenance of blocking.
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  • Ryuichi Kawamura
    1998 Volume 76 Issue 6 Pages 1009-1027
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    A significant coupling of the Asian summer monsoon and ENSO was examined using the NCEP/NCAR reanalysis for the period 1973-1995. Results show that a monsoon index, which is defined as meridional gradient of summertime upper-tropospheric thickness (200-500hPa) anomalies across 20°N over the Indian subcontinent, is highly correlated with Niño-3 SST anomalies in the preceding spring. This is strongly suggestive of the presence of the indirect impact of anomalous SST forcing associated with ENSO on the Asian summer monsoon.
    Due to attenuated Walker circulation in response to a warm episode, convection is suppressed over the northern tropical Indian Ocean and the maritime continent from the preceding winter to spring. The suppressed tropical convection in the preceding spring generates anomalous cyclonic circulation to the west of the Tibetan Plateau as a result of the Rossby-type response to convective heating off the equator. The convection-induced anomalous cyclonic circulation accompanied by large-scale ascending atmospheric motion contributes substantially to increased rainfall and greater soil moisture, thus resulting in decreased land-surface temperature over central Asia to the northwest of the Indian subcontinent. On the other hand, warm SST anomalies are initially introduced over the tropical Indian Ocean in late spring prior to the onset of the monsoon due to the changes in the surface heat flux and/or dynamic response of the ocean to wind forcing, in intimately association with pronounced in situ low-level northeasterly wind anomalies and less cloud cover. Both these different physical processes in the land and ocean areas are crucially responsible for reduced land-ocean thermal contrast (or reduced meridional tropospheric temperature gradient), eventually bringing about the weakening of the Asian summer monsoon. The reverse situation is quite true for strong monsoon years. Once the summer monsoon becomes weak (strong) at its early stage due to these processes, the initially induced warm (cool) SST anomalies over the tropical Indian Ocean are further intensified.
    The mechanism proposed here is valid during the period from the late 1970s to the early 1990s when weak and strong monsoon years are categorized. During that period, the unusual Niño-3 SST anomalies tend to persist from the preceding winter until summer, hence serving as a bridge between the ENSO prevailing in the preceding winter and anomalous summer monsoon. However, regardless of when the monsoon-ENSO coupling is prominent, both the springtime outgoing longwave radiation and low-level wind anomalies dominating over the tropical Indian Ocean, associated with anomalous Walker circulation, are still crucial factors in terms of the potential predictability of the Asian summer monsoon.
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  • K. Sakai, W. R. Peltier
    1998 Volume 76 Issue 6 Pages 1029-1044
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    A previously developed two-dimensional-basin model of the global thermohaline circulation has been asynchronously coupled to an atmospheric energy balance model in support of analyses of the low frequency variability of the climate system. The coupled model, which has previously delivered successful simulations of the Dansgaard-Oeschger oscillations revealed in deep ice-core isotopic data from Summit, Greenland, is herein applied to the last deglaciation event of the current ice age in order to investigate the response of the climate system to transient meltwater forcing. The model employs hydrological forcing functions that consist of two components, one that is related to sea level and constrained by coral-based records of LGM to present sea level history, and a second that is unrelated to sea level and is assumed to exist because of the existence of the continental ice sheets that bounded the region of the North Atlantic basin where deep water is today. Our results show that the model successfully explains the occurrence of a Younger-Dryas-like cool period regardless of the detailed properties of the sea level-related meltwater event that is observed to have followed this millennium-long return to glacial conditions. In order to successfully explain the occurrence of the Bølling/Allerød warm period that occurred prior to the Y-D, however, the model requires the action of the additional “background” anomaly that is unrelated to sea level. We explore the impact on climate response of the properties of these two components of the anomalous forcing.
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  • Tomoaki Ose
    1998 Volume 76 Issue 6 Pages 1045-1063
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    The Asian summer monsoon circulation, especially its climatological seasonal change, was studied as the model response to the prescribed zonal mean field and the prescribed diabatic heat source from the observation. The obtained results are summarized as follows.
    (1) During the Asian summer monsoon season, the prescribed deep heat sources in the southern part of Asia form the Tibetan High, the monsoon trough, the low-level circulation over South Asia, and furthermore, the downward motion in the western part of the Eurasian Continent. The heat sources near the surface over central Asia also induce downward motions aloft.
    (2) In early summer (June), the deep heat sources in the southern part of Asia tend to form southwesterly low-level flows and upward motion southeast of Japan. Those are considered to be the background for the Baiu formation in East Asia as well as heat lows produced in the southern part of Asia. The mid-latitude heat sources associated with the Baiu precipitation produce a low-level jet south of that.
    (3) Climatological seasonal change from early summer (June) to mid-summer (July) is characterized by an air temperature increase in the whole Northern Hemisphere and a northward shift of a weakened westerly jet. When in the model a zonal mean field in June is replaced by that in July, the major characteristics of the seasonal change are obtained qualitatively; low-level jets and upward motion areas in South Asia and East Asia shift from the ocean side of the coasts toward the land side. This change of vertical motion is consistent with the seasonal change of deep heat sources from June to July.
    (4) The climatological seasonal change from mid-summer (July) to late summer (August) is characterized by enhanced convective activity in the extended area of the subtropical western Pacific. When deep heat sources in July are replaced by those in August over the western Pacific only, the major characteristics of the seasonal change over the Pacific and the Indian Ocean are obtained. The expansion of the Tibetan High at the upper-level and the Pacific High at low-level over Japan is also simulated by the seasonal change of the western Pacific heat sources only.
    (5) The model simulation with the combination of the diabatic heat source for August and the zonal mean field for June is compared with the climatological August simulation. It is indicated that the zonal mean field delayed from its seasonal migration could be related to weak monsoon circulation and the associated precipitation anomalies in the mid-latitudes and the subtropics.
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  • Kunimoto Iwai, Masakatu Kato
    1998 Volume 76 Issue 6 Pages 1065-1069
    Published: December 25, 1998
    Released: September 15, 2009
    JOURNALS FREE ACCESS
    Thermal relaxation times (TRTs) of initially warm and cool waterdrops with radii larger than 1.7mm falling at terminal velocity in air were measured under conditions of about 20°C. The temperatures of both waterdrops exponentially approach the wet bulb temperature of environmental air. The TRTs of both waterdrops linearly increase with increasing radius. The TRTs of cool waterdrops are larger than those of warm ones. The relationship between the TRTs (τ) of cool and warm waterdrops and their equlvalent radii (a) larger than 1.7mm are respectively expressed as the following formulas: τcool=3.82a-2.76, τwarm=2.37a-0.20 (a is given in mm).
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